Truth is, Russia and Islamic State have different projects going in Syria, projects that don’t even overlap much. Syria is more full of bad projects than the ninth-grade Metal Shop class where they set my jacket on fire with a soldering iron (while I was wearing it). That place was full of projects thought up by adolescent psychopaths, all designed to kill or maim, and mostly ineffective.
Which, come to think of it, is not a bad description of Syria at the moment. For a smallish country, Syria has more theaters of war going than a multiplex doing a Private Ryan marathon. The Kurds of YPG/PKK have their own project going in the north, along the Turkish border. The Alawites are trying to survive and carve a rump state for themselves in the coastal hills. The Christians have executed a simple plan: “get the Hell out of here while we can.” Hezbollah’s project could be summed up as, “Ugh, I guess we gotta help these weak-ass Alawites after all, damn it.” Israel’s project is “Attack Hezbollah nonstop, but never touch the Sunni militias because they’re not a real threat.” Jabhat-an-Nusra, Ahrar-as-Sham, and the other Sunni militias are competing for ownership of the inland Sunni state they hope will come out of this chaos.
Here's the straight dope: Russia wants to protect their oil assets in the region, and Russia wants to protect their political assets in the region so that they can eventually gain access to a warm water port (that's not, you know, in the black sea) It's the exact same reason that Russia invaded Afghanistan, why the USSR held so many of the 'Stans. My brother, a history / Poli-sci major, once joked that America has a 4 year plan to take over the world, Russia has a 400 year plan.
Anyone interested in Russia's foreign policy in the area should have a look at War at the Top of the World, by Eric Margolis. It focuses more specifically on the worldwide actions in Afghanistan, Pakistan, Kashmir, etc, but the concepts also apply here and elsewhere in the middle east and Asia.